Brazil arrested ten people suspected of plotting a terror attack inspired by the so-called Islamic State during the Olympic Games, the justice minister said.
Alexandre de Moraes said that police acted to smash the ring after they began discussing using weapons to launch an attack during the Rio games.
A further two people were still being hunted, he added.
No specific target for an attack was said to have been identified by the group, all of whom reportedly pledged support to IS via social media.
Those involved participated in an online group denominated 'the defenders of Sharia' and were planning to acquire weapons to commit crimes in Brazil and even overseas.
He described the group as "absolutely amateur" with no real preparation for an attack and no known contact with the IS terror group despite their pledges of allegiance.
It comes after a previously unknown Brazilian group calling itself "Ansar al-Khilafah Brazil" announced its backing for IS via the messaging app Telegraph on Sunday.
Security services have stepped up security in preparation for the Olympics, which are set to start in just over two weeks on August 5.
Six people held in connection with a foiled terrorist attack on a train travelling between Amsterdam and Paris have been released without charge, police said.
Gunman Ayoub El-Khazzani, 26, was overpowered by fellow passengers after they noticed him loading a weapon on the high-speed Thalys service last August.
He was charged with attempted murder of a terrorist nature and remanded in custody.
Six others who were held for questioning on Monday have now been released, a spokesman for the Belgian federal prosecutor said.
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